Articles comprising cemented carbides are commonly used in applications that involve high stresses and friction, such as cutting tools or cutting inserts for use in turning, milling, and drilling; seal rings for agitators and pumps; and rolls for rolling steel. Articles comprising cemented carbides tend to fail by thermal cracking. Cracks in such articles may be initiated if the article is heated above a threshold value, and the cracks may further propagate if the article is subject to thermal cycling.
For example, earth boring (or drilling) bits are commonly employed for oil and natural gas exploration, mining, and excavation. Such earth boring bits may have fixed or rotatable cutting elements. FIG. 1 illustrates a typical rotary cone earth boring bit 10 with rotatable cutting elements 11. Cutting inserts 12, typically made from a cemented carbide, are placed in pockets fabricated on the outer surface of the cutting elements 11. Several cutting inserts 12 may be fixed to the rotatable cutting elements 11 in predetermined positions to optimize cutting.
The service life of an earth boring bit is typically a function of the wear properties of the cemented carbide inserts. One way to increase earth boring bit service life is to employ cutting inserts made of materials with improved combinations of strength, toughness, and abrasion/erosion resistance. As stated above, the cutting inserts comprise cemented carbides, a type of cemented hard particle. The choice of cemented carbides for such applications is predicated on the fact that these materials offer very attractive combinations of strength, fracture toughness, and wear resistance (i.e., properties that are extremely important to the efficient functioning of the boring or drilling bit). Cemented carbides are composites comprising a dispersed, discontinuous phase including particles of carbides of one or more of the transition metals belonging to groups IVB, VB, and VIB of the periodic table (Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W), and a continuous binder phase (typically including cobalt, nickel, or iron) cementing together the hard particles. Among the different possible hard particle-binder combinations, cemented carbides based on tungsten carbide (WC) as the hard particle and cobalt as the binder phase are the cemented hard particles most commonly employed.
The properties of cemented carbides depend upon, among other properties, two microstructural parameters, namely, the average hard particle grain size and the weight or volume fraction of the hard particles and/or the binder. In general, the hardness and wear resistance increases as the grain size decreases and/or the binder content decreases. On the other hand, fracture toughness increases as the grain size increases and/or as the binder content increases. Thus, there is a trade-off between wear resistance and fracture toughness when selecting a cemented carbide grade for any application. As wear resistance increases, fracture toughness typically decreases, and vice versa.
FIGS. 2A-2E illustrate some of the different shapes and designs of the cemented carbide inserts typically employed in rotary cone earth boring bits. Cutting inserts for earth boring bits are typically characterized by the shape of the domed portion, such as, ovoid (FIG. 2A), ballistic (FIG. 2B), chisel (FIG. 2C), multidome (FIG. 2D), and conical (FIG. 2E). The choice of the shape and cemented carbide grade employed depends upon the type of rock to be drilled. Regardless of shape or size, all inserts have a working portion in the form of a cutting portion, and a body portion. For example, cutting insert 20 in FIG. 2A includes dome-shaped cutting portion 22 and body portion 24. Also, for example, cutting insert 30 in FIG. 2B includes ballistic-shaped cutting portion 32 and body portion 34. The cutting action is performed by the cutting portion, while the body portion provides support for the cutting portion. Most, or all, of the body portion is embedded within the bit body or cutting element, and the body portion is typically inserted into the bit body by press fitting the cutting insert into a pocket.
As previously stated, the cutting action is primarily provided by the cutting portion of the tool. The first portion of the cutting portion to begin wearing away is the top half and, in particular, the extreme tip of the cutting portion. In the case of earth boring bits, as the top of the cutting portion begins to flatten out, the efficiency of cutting decreases dramatically since the earth is being removed more by a rubbing action, as opposed to a more efficient cutting action. As rubbing action continues, considerable heat may be generated by the increase in friction between the rock and the cutting insert, thereby resulting in heating of portions of the insert. If the temperature of any portion of the article exceeds a threshold valve, cracks will be initiated at the interface of the hard particles and the binder. Thermal cycling of the article causes propagation of the cracks.
Accordingly, there is a need for improved cemented carbide cutting inserts for earth boring bits having increased resistance to thermal fatigue and cracking. More generally, there is a need for improvements to articles including a working portion including cemented carbide that may be subject to cracking caused by thermal cycling.